Assalamu alaikum sheikh,
Saying “Rabbighfirli” once is wajib in sitting between prostrations. I have found it is recommended to say three times however I cannot find a hadith to support this. There is a hadith to support “Rabbighfirli” twice. What is the proof for saying it three times?
Also, would it suffice according to the madhab to say “Allahummaghfirl” instead? I have found this written in “Ahmad Ibn Hanbali Treatise on Prayer” however the fiqh books do not make mention of it.
The evidences show that one should say (رب اغفر لي) between prostrations, so to say this once is the minimum. As for the narration reported by al-Nasa’i that mentions it twice, then this does not mean that a limit was placed to two, but rather that the Messenger (صلى الله عليه وسلم) repeated this dhikr; not that he only said it twice. And since three times is the recommended number for many adhkar during salah, then consistency dictates that it is the recommended number here too.
If someone said (اللهم اغفر لي) then this would also suffice them, because the difference is of not much significance, and there are reports that mention this wording too, reported by Abu Dawud, al-Tirmidhi, and others. So there is flexibility in this regard.
And success is from Allah